This invention relates to a fading simulation method and a fading simulator.
Digital radio communication sometimes suffers from an error caused by distortion in waveform arising from selective fading wherein the level drops in a particular frequency since it employs frequencies of a band wider than that of FM (frequency modulation) communication. Therefore, it is necessary to confirm a performance of a radio equipment with regard to fading in advance by way of testing, and to this end, a fading simulator is used in such a manner as seen in FIG. 14. Referring to FIG. 14, a fading simulator 82 is interposed between a transmitter 81 and a receiver 83. A transmission signal from the transmitter 81 is inputted to the fading simulator 82, in which waveform distortion similar to that which is caused by fading is applied to the transmission signal to artificially cause fading. The output of the fading simulator 82 is received by the receiver 83 thereby to conduct a performance test of the radio system with regard to fading.
FIG. 15 is a block diagram showing a general construction of such fading simulator. Referring to FIG. 15, the fading simulator includes a distributor 91 for distributing an input signal. A first path 201 is formed from a circuit from the distributor 91 to a composer 98 by way of a first variable attenuator 92, a delay compensator 93 and a delay element 94 while a second path 202 is formed from another circuit from the distributor 91 to the composer 98 by way of an infinite phase shifter 95, a second variable attenuator 96 and another delay element 97.
Here, the first variable attenuator 92 attenuates the amplitude of a signal passing the first path 201, and the delay compensator 93 and the delay element 94 set a delay time for a signal passing the first path 201.
The infinite phase shifter 95 varies the phase of a signal passing the second path 202 to vary the notch frequency of fading. The second variable attenuator 96 attenuates the amplitude of a signal passing the second path 202. The delay element 97 sets the delay time of a signal passing the second path 202.
The composer 98 composes signals from the first and second paths 201 and 202. The fading simulator shown in FIG. 15 further includes a variable attenuator 99 for attenuating the amplitude of a signal composed by the composer 98 to vary the gain between the input and the output of the fading simulator.
The fading simulator further includes a control section 100 which includes phase control means 101, first amplitude control means 102, second amplitude control means 103 and level control means 104.
The phase control means 101 controls the infinite phase shifter 95 to control the phase of a signal passing the second path 202.
The first amplitude control means 102 controls the amount of attenuation of the first variable attenuator 92 to control the amplitude of a signal passing the first path 201. The second amplitude control means 103 controls the amount of attenuation of the second variable attenuator 96 to control the amplitude of a signal passing the second path 202.
The level control means 104 controls the amount of attenuation of the amplitude of a signal at the variable attenuator 99.
In the fading simulator of the construction described above, an input signal is distributed to the first and second paths 201 and 202 by the distributor 91. In the first path 201, the input signal is first attenuated in amplitude by the first variable attenuator 92, which is controlled by the first amplitude control means 102, and then delayed by the delay compensator 93 and the delay element 94. Meanwhile, in the second path 202, the input signal is first varied in phase by the infinite phase shifter 95, which is controlled by the phase control means 101, and then attenuated in amplitude by the second variable attenuator 96, which is controlled by the second amplitude control means 103, whereafter it is delayed by the delay element 97. The composer 98 composes signals from the first and second paths 201 and 202, and the variable attenuator 99 attenuates the amplitude of an output signal of the composer 98 to vary the gain between the input and the output of the fading simulator under the control of the level control means 104.
With the fading simulator of the construction described above, however, when a wide band is used, since an infinite phase shifter is used as a phase shifter, the second path 202 exhibits such a frequency amplitude characteristic as seen from the waveform of FIG. 16(b), which is different from such a flat characteristic as of a frequency amplitude characteristic of the first path 201 shown in FIG. 16(a). Consequently, there is a problem in that the frequency amplitude characteristic of the second path 202 described above varies the amount of attenuation when the notch frequency sweeps.
More particularly, where the difference in delay time between the first and second paths 201 and 202 is represented by .tau..sub.1, the amplitude ratio by .rho..sub.1, the phase difference by .theta..sub.1 and the amplitude of the first path 201 by A, the output amplitude characteristic A(.omega.) is given by the following equation: EQU A(.omega.)=A(1+2.rho. cos (2.pi.f.tau..sub.1 +.theta..sub.1)+.rho..sub.1.sup.2).sup.1/2
where f is a frequency.
Further, the condition for A(.omega.) to be minimized (for the notch attenuator amount to be maximized) is that 2.pi.f.tau..sub.1 +.theta..sub.1 =.pi., and in this instance, A(.omega.)=A((1-.rho.).sup.2).sup.1/2 =A(1-.rho.), and the attenuation ratio is given by A(.omega.)/A=1-.rho.. Further, if this is converted into dB (decibel), then 20log(A(.omega.)/A)=20log(1-.rho.).
If, for example, .rho.=0.97 (-0.26 dB), then as indicated by k in the waveform diagram of FIG. 16(c), the attenuation amount at a notch point f.sub.0 is given by 20log(1-0.97)=-30 dB.
Then, if .theta..sub.1 is varied so that the notch point is adjusted to f=f.sub.0 -.DELTA.f, then the amplitude characteristic of the second path 202 changes, when -1 dB from the point f.sub.0 as seen from FIG. 16(b), to -0.26 dB -1 , dB=-1.26 dB, that is, .rho.=0.86, and the attenuation amount then changes to 20log(1-0.86)=-17 dB as seen from j in FIG. 16(c). A similar subject occurs with an alternative case wherein the notch point is varied to f=f.sub.0 +.DELTA.f.